1. Field of the Invention
The present invention relates to a compact variable focal length optical system and, more particularly, to a variable focal length system in which the exit pupil position is located at a far position with respect to the total lens length.
2. Related Background Art
In recent years, as lens-shutter cameras, digital still cameras, and the like, cameras with zoom lenses are popular, and in particular, camera having so-called zoom lenses with high zoom ratios of 3.times. or higher are becoming increasingly popular.
Such zoom lens with a high zoom ratio uses, in many cases, a so-called multi-unit zoom lens having three or more lens units which are movable when the focal length of the lens system is varied. Various proposals have been made concerning zoom lenses which have a field angle up to about 60.degree..
In these cameras, since a lens and a camera body are integrated unlike in lens exchangeable type single-lens reflex cameras, a size reduction of the lens system leads to that of the camera body, and various proposals have been made in association with zoom lenses suitable for a size reduction.
A zoom lens for a digital still camera uses a CCD as a photo sensor. The CCD has a microlens array immediately before its light-receiving element to enhance the collecting power. When the exit pupil position of the lens system is near the CCD, since a light beam that is to reach the peripheral portion of the frame is incident on the microlens array obliquely with respect to the optical axes of the respective lens surfaces of the microlens array, the light beam is focused by the microlens array at a position different from the light-receiving element. For this reason, the light amount becomes insufficient on the peripheral portion of the frame, and consequently, the optical design is restricted in that the exit pupil position of the lens system must be separated from the image plane.
For example, in an image-side telecentric optical system in which the exit pupil position is set at infinity position, the image position (i.e., exit pupil position) of a stop formed by a lens system located on the image side of the stop is located at infinity position. In order to shorten the distance from the stop to the CCD in such image-side telecentric optical system, the lens system on the image side of the stop must have a strong convergence effect, and the thickness, in the optical axis direction, of the lens system itself must be decreased. For this reason, such optical system is not suitable for a decrease in total lens length.